A　fractured　bolt　connecting　the　bearing　and　the　stiffening　girder　of　a　half-through　steel-arch　bridge,　with　a　main　span　length　of　113　m,　which　was　opened　for　public　in　September　1996,　was　found　in　October　2013　during　periodic　visual　inspection.　Cracks　were　also　found　in　the　bottom　flange　of　the　stiffening　girder　near　the　four　corners　of　the　sole　plate　in　January　of　2014,　when　the　bolts　were　replaced.　In　order　to　assess　the　main　causes　of　the　cracks,　fatigue　failure　analysis　by　dynamic　testing　and　monitoring　of　strain　conditions,　temperature　of　the　stiffening　girder,　and　rotation　of　the　bearing　were　carried　out.　Additionally,　finite-element　(FE)　analysis　was　performed　to　provide　further　insight　on　the　causes　of　the　cracks.　The　results　showed　that　bending　moments　due　to　axial　forces　in　the　stiffening　girder,　caused　by　longitudinal　displacement　of　the　girder　under　vehicle　load,　are　the　main　cause　of　the　cracks　and　bolt　fracture.　©　2019　American　Society　of　Civil　Engineers.